Redispersible polymer powders are polymer emulsions which have been converted by spray drying (atomizing the aqueous polymer dispersions in a stream of hot air) to free flowing powders. When blended with water, these powders redisperse back into liquid emulsions with essentially identical properties to the original emulsions. With dispersions whose polymers have glass transition temperatures (Tg) below about 50.degree. C., it is necessary to add dispersing aids to coat the individual spray dried particles to prevent them from sticking together and to promote redispersion of the particles into an emulsion. The sticking can occur in the spray drying apparatus when the powder is at elevated temperatures, or upon long term storage under the influences of heat, humidity and pressure.
The current technology in the area of dispersing aids involves mainly the use of polyvinyl alcohol (PVOH). PVOH works well in many redispersible powders that are commercially available. It appears to work as a dispersing aid only with polymer emulsions that utilize PVOH as a colloidal stabilizer, i.e., the emulsions themselves are produced using PVOH as the colloidal stabilizing system to produce a stable emulsion. The use of PVOH as the colloidal stabilizer generally produces an emulsion with a high degree of water sensitivity. This built in water sensitivity, along with the addition of PVOH, allows these emulsions to be redispersible and, consequently, spray dried.
However, there are many polymer emulsions that utilize other colloidal stabilizing systems. These other stabilizing systems include polyvinyl pyrrolidone (PVP), cellulosics, surfactants and combinations of these with and without PVOH. In general, polymer emulsions produced with stabilizing systems other than PVOH are not redispersible through the addition of PVOH as a dispersing aid.
There are advantages in using these other colloidal stabilizing systems. They generally produce polymer emulsions with better water resistance and a finer particle size than a PVOH stabilizing system. When a polymer emulsion with superior water resistance is used in a formulation, as for example, an adhesive or an additive into cement, the resulting final product will generally also have superior water resistance.
There are also other advantages to the cellulosic and surfactant stabilized systems. There are certain monomer systems that cannot be stabilized with PVOH alone. This severely limits the types of polymer emulsions that can be spray dried. It is in these areas that PVOH is deficient as a dispersing aid.
There are several disclosures regarding the use of vinyl pyrrolidone polymers as a dispersing aid in redispersible polymer powders:
DE 22 38 903 discloses drying a mixture of aqueous dispersions of copolymers using as dispersing aids a copolymer (A) of 20 to 80 wt% vinyl chloride and 80 to 20 wt% vinyl acetate and/or vinyl propionate, a copolymer (B) of 60 to 90 wt% styrene, 5 to 35 wt% of acrylonitrile, methacrylonitrile, methyl acrylate, ethyl acrylate, t-butyl acrylate, methyl methacrylate, vinyl acetate and/or vinyl propionate, 5 to 10 wt% of an .alpha.,.beta.-olefin-unsaturated carboxylic acid, and 0 to 5 wt% of acrylamide, methacrylamide and/or vinyl pyrrolidone, and if applicable also a vinyl pyrrolidone polymer (C), with a ratio of copolymer (A) to copolymer (B) to polymer (C) is 75 to 95:5 to 25:0 to 20. Example 2 shows the use of PVP in the spray drying a blend of copolymer emulsions, one of which was prepared using PVOH as part of a costabilizing system.
U.S. Pat. No. 28,780 discloses a process for the preparation of free-flowing, lump-free redispersible synthetic resin powders by spray drying aqueous dispersions of synthetic resins containing an added amount of a water soluble condensation from melamine and fomaldehyde and containing sulfonate groups. It is also disclosed at Col 3/45-53 that "[i]t may also be advantageous, optionally to add, to the dispersion before spray drying protective colloids, such as polyvinyl alcohol, cellulose derivatives and polyvinyl pyrrolidone, or inert substances, such as kaolin, chalk, silicates, talc, diatomaceous earth, silicic acid, barium or calcium sulfate, in amounts of from 5% to 50% by weight, based on the solids content of the synthetic resin powder. Thereby in some cases improvements in the storage ability are also attained." U.S. Re '780 does not suggest the use of PVP alone and none of the examples show the use of PVP.
EP 0 078 449A discloses a process for the production of water dispersible polymer powders by spraying the aqueous polymer dispersions containing an admixture of:
a) a water soluble copolymer consisting of 20 to 70 wt% vinyl pyrrolidone and 30 to 80 wt% vinyl acetate and/or PA0 b) a water soluble alkali- and/or alkali earth salt of a naphthalene sulfonic acid fomaldehyde condensate product.
This reference specifically discloses the use of the vinyl pyrrolidone/vinyl acetate copolymer for polymer emulsions down to a Tg of -60.degree. C. It does not suggest the use of PVP homopolymer.
CA 2,020,449 discloses redispersible polymer powders obtainable by atomizing aqueous polymer dispersions to which from 3 to 50 wt%, based on the polymer, of a water soluble alkali metal salt or alkaline earth metal salt of a phenolsulfonic acid/fomaldehyde condensate has been added. This reference at page 4/31-36 states, "In addition to the condensates, water-insoluble finely divided solids, for example finely divided silica or water-soluble protective colloids, for example polyvinyl alcohols or vinyl pyrrolidone (co)polymers, may also have been added to the polymer dispersions in the preparation of the polymer powders." Again this reference does not suggest the use of PVP alone and none of the examples show the use of PVP.